Lithographic apparatus, device manufacturing method and radiation system

ABSTRACT

A lithographic projection apparatus includes an illumination system configured to provide a beam of radiation; a support configured to support a patterning device, the patterning device configured to impart the beam of radiation with a pattern in its cross section; a substrate table configured to hold a substrate, and a projection system configured to project the patterned beam of radiation onto a target portion of the substrate, wherein the illumination system has a radiation source and at least one mirror configured to enhance an output of the source. The illumination system may include a second radiation source and at least one mirror positioned between the radiation sources to image the output of the second source onto the first source, thereby enhancing the output of the source. The radiation sources may be operable to emit radiation in the EUV wavelength range.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a Continuation-In-Part of U.S. application Ser. No.10/844,577, filed May 13, 2004, now U.S. Pat. No. 7,105,837 the entirecontents of which are herein incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a lithographic apparatus, a devicemanufacturing method and a radiation system. The present inventionrelates to a lithographic apparatus designed for use with radiationhaving a wavelength in the extreme ultraviolet (EUV) range.

2. Description of the Related Art

A lithographic apparatus is a machine that applies a desired patternonto a target portion of a substrate. Lithographic apparatus can beused, for example, in the manufacture of integrated circuits (ICs). Inthis case, a patterning device, which is alternatively referred to as amask or a reticle, may be used to generate a circuit patterncorresponding to an individual layer of the IC. This is done using aprojection system that is between the reticle and the substrate and isprovided to image an irradiated portion of the reticle onto a targetportion of a substrate. The projection system includes components todirect, shape and/or control a beam of radiation. The pattern can beimaged onto the target portion (e.g. including part of one, or several,dies) on a substrate, for example a silicon wafer, that has a layer ofradiation-sensitive material, such as resist. In general, a singlesubstrate contains a network of adjacent target portions that aresuccessively exposed. Known lithographic apparatus include so-calledsteppers, in which each target portion is irradiated by exposing anentire pattern onto the target portion at once, and so-called scanners,in which each target portion is irradiated by scanning the patternthrough the projection beam in a given direction, usually referred to asthe “scanning” direction, while synchronously scanning the substrateparallel or anti-parallel to this direction.

An important aspect in lithography is the size of features of thepattern applied to the substrate. It is desirable to produce apparatuscapable of resolving features as small and close together as possible. Anumber of parameters affect the available resolution of features. One ofthese is the wavelength of the radiation used to expose the pattern.Using radiation with an EUV wavelength between 5 and 20 nm, andtypically 13.5 nm, it is anticipated that it will be possible tomanufacture feature sizes down to 32 nm.

Various EUV sources are known, for example some plasma-based radiationsources emit radiation in this wavelength range. These sources arevolume radiators. By this it is meant that the radiator is (virtually)transparent for the radiation it emits and so the radiation producedinside the volume propagates freely towards the surface of the volume,and passes this surface without interacting with the radiating species.Plasma sources can be stimulated either by using suitable laserradiation or by using an electrical discharge. These sources come inmany different forms, and are well known in the art. Various examplesare described in WO 01/99143.

While using wavelengths in the EUV range allows for the fabrication ofvery small features, it can cause practical problems. Radiation at thiswavelength is absorbed in all materials and is therefore not suitablefor use with refractive optics. The optics in a projection system foruse with EUV lithography must therefore be based on mirrors, which canonly operate in an ultra high vacuum (UHV) environment. A furtherproblem is that the conversion efficiency, i.e. the ratio of power outat the required wavelength to power in, for discharge sources is verylow, which means that the radiation power output is correspondingly low.

SUMMARY OF THE INVENTION

It is an aspect of the present invention to alleviate, at leastpartially, the problems discussed above.

According to a first aspect of the present invention, there is provideda lithographic projection apparatus including an illumination systemconfigured to provide a beam of radiation; a support configured tosupport a patterning device, the patterning device configured to impartthe beam with a pattern in its cross section; a substrate tableconfigured to hold a substrate, and a projection system configured toproject the patterned radiation onto a target portion of the substrate,wherein the illumination system includes a first radiation source and atleast one mirror configured to increase the power of the beam.

By increasing the radiation power of the beam, it is meant increasingthe power to a level above that which would be available in the absenceof the one or more mirrors.

The radiation source may be a volume radiation source and is open at itsrear or back side, and the mirror is provided behind the radiationsource so that radiation radiated rearwardly of the source is reflectedforwardly to add to the illuminating radiation, thereby increasing theradiation power in the projection beam. The mirror may be a sphericalmirror or an ellipsoidal mirror. The source may be positionedsubstantially at the focal point of the mirror.

Alternatively, or additionally, the at least one mirror may beconfigured to image a second radiation source onto the first radiationsource. The at least one mirror may be provided between the radiationsources to image the second radiation source onto the first radiationsource, thereby to enhance the output of the first radiation source. Forexample, a single ellipsoidal mirror could be used, this beingrotationally symmetric about the optical axis and open at its ends sothat light can pass into and out of it. In this case, the first andsecond sources would be located at first and second focal points of themirror. Alternatively, a combination of two parabloidal mirrors could beused, with the first source being positioned at the focus of the firstparabola, and the second source being positioned at the focus of thesecond parabola. Of course, in this arrangement, one of the mirrors hasto be open at one end so that light can be transmitted outwardly towardsthe next stage of the illumination system. Other configurations includea nested, grazing incidence mirror, such as a Wolter-type mirror, awhispering gallery mirror, and a spherical mirror. By adding together ormultiplexing the outputs of two or more radiation sources using someform of mirror arrangement, the overall radiation power of the beam isincreased.

By using mirrors to image radiation from a source onto itself or toimage a second source onto it, there is provided a simple arrangementfor enhancing the radiation power of the beam without increasing theentendue and without using moving parts.

At least one debris suppression system may be provided between the firstand/or second radiation sources and the at least one mirror. The debrissuppression system may take any suitable form. For example, each suchsystem may include a foil trap and/or a chopper arrangement.

The radiation source may be a volume radiating source. The radiationsource may include a plasma radiation source. The plasma radiationsource may be an electrical discharge source or a laser produced plasmasource. The radiation source may also be operable to emit radiation inthe EUV range. Plasma sources are at least optically transparent betweenradiation pulses.

The at least one radiation source and the at least one mirror may beprovided in a single radiation source unit and may be adapted to befitted to the illumination system.

According to another aspect of the invention, there is provided a devicemanufacturing method including providing a substrate; passing apatterned projection beam of radiation through a projection system so asto project it onto a target portion of the substrate, and increasingradiation power of the projection beam using one or more mirrors in theillumination system.

The method may further involve positioning the mirror behind theradiation source so that radiation radiated rearwardly of the source isreflected forwardly to add to the illuminating radiation. The mirror maybe a spherical or an aspherical mirror, and the source may besubstantially at the focal point of the mirror.

The method may also include providing a second radiation source, andimaging the second radiation source onto the first radiation sourceusing the one or more mirrors, thereby enhancing the output of the firstradiation source. The method may further include providing at least onemirror between the sources to image the second radiation source onto thefirst radiation source. The at least one mirror may be ellipsoidal.

The first and/or second radiation source may be volume radiationsources. The first and/or second radiation sources may be plasmaradiation sources. The plasma radiation source may be an electricaldischarge plasma source or a laser stimulated plasma source. Theradiation source may be perable to emit radiation in the EUV range.

The device manufacturing method may include stimulating the first andsecond sources at the same time so that their outputs are added.Alternatively, the method may include causing the sources to emitradiation alternately, so that their outputs are interlaced. Theabsorption of the radiation emitted by the rearward one of the sourcesby the other source can be avoided.

According to another aspect of the present invention, there is provideda radiation system or unit configured to provide a beam for alithographic projection apparatus, the unit including a radiation sourceand at least one mirror configured to increase the radiation poweroutput of the beam.

The at least one mirror may be provided behind the radiation source sothat radiation radiated rearwardly of the source is reflected forwardlyto add to the illuminating radiation. The mirror may be a spherical oran aspherical mirror, and the source may be positioned substantially atthe focal point of the mirror.

Alternatively, or additionally, the at least one mirror may beconfigured to image a second radiation source onto the first radiationsource. The at least one mirror may be provided between the radiationsources to image the second radiation source onto the first radiationsource, thereby enhancing the output of the first radiation source. Forexample, a single ellipsoidal mirror could be used, this beingrotationally symmetric about the optical axis and open at its ends sothat light can pass into and out of it. In this embodiment, the firstand second sources would be located at first and second focal points ofthe mirror. Alternatively, a combination of two parabloidal mirrors maybe used, with the first source being positioned in the focus of thefirst parabola, and the second source being positioned at the focus ofthe second parabola. Other configurations include a nested, grazingincidence mirror, such as a Wolter-type mirror, a whispering gallerymirror, and a spherical mirror.

The radiation source may be a volume radiating source. The radiationsource may be a plasma radiation source. The plasma radiation source maybe an electrical discharge source or a laser stimulated plasma source.The radiation source may be operable to emit radiation in the EUV range.

According to another aspect of the present invention, a device is madedirectly or indirectly using the lithography system and/or devicemanufacturing method and/or radiation unit of any of the precedingaspects of the invention.

The terms “radiation” and “beam” used herein encompass all types ofelectromagnetic radiation, including ultraviolet (UV) radiation (e.g.having a wavelength of 365, 248, 193, 157 or 126 nm) and extremeultraviolet (EV) radiation (e.g. having a wavelength in the range of5-20 nm), as well as particle beams, such as ion beams or electronbeams.

The term “patterning device” used herein should be broadly interpretedas referring to a device that can be used to impart to a beam ofradiation a pattern over its cross-section such as to create a patternin a target portion of the substrate. It should be noted that thepattern imparted to the projection beam may not exactly correspond tothe desired pattern in the target portion of the substrate. Generally,the pattern imparted to the projection beam will correspond to aparticular functional layer in a device being created in the targetportion, such as an integrated circuit.

The patterning device may be transmissive or reflective. The patterningdevice may include any of masks, programmable mirror arrays, andprogrammable LCD panels. Masks are well known in lithography, andinclude mask types such as binary, alternating phase-shift, andattenuated phase-shift, as well as various hybrid mask types. An exampleof a programmable mirror array employs a matrix arrangement of smallmirrors, each of which can be individually tilted so as to reflect anincoming radiation beam in different directions; in this manner, thereflected beam is patterned.

The support, e.g. bears the weight of, the patterning device. It holdsthe patterning device in a way depending on the orientation of thepatterning device, the design of the lithographic apparatus, and otherconditions, such as for example whether or not the patterning device isheld in a vacuum environment. The support can use mechanical clamping,vacuum, or other clamping techniques, for example electrostatic clampingunder vacuum conditions. The support be a frame or a table, for example,which may be fixed or movable as required and which may ensure that thepatterning device is at a desired position, for example with respect tothe projection system. Any use of the terms “reticle” or “mask” hereinmay be considered synonymous with the more general term “patterningmeans”.

The term “projection system” used herein should be broadly interpretedas encompassing various types of projection system, including refractiveoptical systems, reflective optical systems, and catadioptric opticalsystems, as appropriate for example for the exposure radiation beingused, or for other factors such as the use of an immersion fluid or theuse of a vacuum. Any use of the term “lens” herein may be considered assynonymous with the more general term “projection system”.

The illumination system may also encompass various types of opticalcomponents, including refractive, reflective, and catadioptric opticalcomponents to direct, shape, and/or control the beam of radiation, andsuch components may also be referred to below, collectively orsingularly, as a “lens”.

The lithographic apparatus may be of a type having two (dual stage) ormore substrate tables (and/or two or more mask tables). In such“multiple stage” machines the additional tables may be used in parallel,or preparatory steps may be carried out on one or more tables while oneor more other tables are being used for exposure.

The lithographic apparatus may also be of a type wherein the substrateis immersed in a liquid having a relatively high refractive index, e.g.water, so as to fill a space between the final element of the projectionsystem and the substrate. Immersion liquids may also be applied to otherspaces in the lithographic apparatus, for example, between the mask andthe first element of the projection system. Immersion techniques arewell known in the art for increasing the numerical aperture ofprojection systems

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the present invention will now be described, by way ofexample only, with reference to the accompanying schematic drawings inwhich corresponding reference symbols indicate corresponding parts, andin which:

FIG. 1 is a schematic diagram of a lithographic apparatus according tothe present invention;

FIG. 2 is a more detailed view of parts of the apparatus of FIG. 1;

FIG. 3 is a schematic diagram of an illumination source for use in theapparatus of FIG. 2;

FIG. 4 is a schematic diagram of another illumination source for use inthe apparatus of FIG. 2;

FIG. 5 is a schematic diagram of yet another illumination source for usein the apparatus of FIG. 2, and

FIG. 6 is a schematic diagram of still another illumination source foruse in the apparatus of FIG. 2.

DETAILED DESCRIPTION

FIG. 1 shows an apparatus including an illumination system (illuminator)IL configured to provide a beam PB of radiation (e.g. UV or EUVradiation). A first support (e.g. a mask table) MT is configured tosupport a patterning device (e.g. a mask) MA and is connected to a firstpositioning device PM that accurately positions the patterning devicewith respect to a projection system (“lens”) PL. A substrate table (e.g.a wafer table) WT is configured to hold a substrate (e.g. aresist-coated wafer) and is connected to a second positioning devicethat accurately positions the substrate with respect to the projectionsystem PL. The projection system (e.g. a reflective projection lens) PLis configured to image a pattern imparted to the beam PB by thepatterning device MA onto a target portion C (e.g. including one or moredies) of the substrate W. The apparatus of FIG. 1 is of a reflectivetype, for example employing a reflective mask or a programmable mirrorarray of a type as referred to above. However, it will be appreciatedthat the apparatus may be of a transmissive type, for example employinga transmissive mask.

The illuminator IL receives radiation from a radiation source SO. Thesource and the lithographic apparatus may be separate entities, forexample when the source is a plasma discharge source. In such cases, thesource is not considered to form part of the lithographic apparatus andthe radiation is generally passed from the source SO to the illuminatorIL with the aid of a radiation collector, including for example suitablecollecting mirrors and/or a spectral purity filter. In other cases, thesource may be integral part of the apparatus, for example when thesource is a mercury lamp.

The illuminator IL may include an adjusting device configured to adjustthe angular intensity distribution of the beam. Generally, at least theouter and/or inner radial extent (commonly referred to as σ-outer andσ-inner, respectively) of the intensity distribution in a pupil plane ofthe illuminator can be adjusted. The illuminator provides a conditionedbeam of radiation PB having a desired uniformity and intensitydistribution in its cross-section. This beam PB is incident on the maskMA, which is held on the mask table MT. Being reflected by the mask MA,the beam PB passes through the projection system PL, which focuses thebeam onto a target portion C of the substrate W. With the aid of thesecond positioning device PW and a position sensor IF2 (e.g. aninterferometric device), the substrate table WT can be moved accurately,e.g. so as to position different target portions C in the path of thebeam PB. Similarly, the first positioning device PM and a positionsensor IF1 (e.g. an interferometric device) can be used to accuratelyposition the mask MA with respect to the path of the beam PB, e.g. aftermechanical retrieval from a mask library, or during a scan. In general,movement of the object tables MT and WT will be realized with the aid ofa long-stroke module (coarse positioning) and a short-stroke module(fine positioning), which form part of the positioning devices PM andPW. However, in the case of a stepper (as opposed to a scanner) the masktable MT may be connected to a short stroke actuator only, or may befixed. Mask MA and substrate W may be aligned using mask alignment marksM1, M2 and substrate alignment marks P1, P2.

The depicted apparatus can be used in various modes. For example, instep mode, the mask table MT and the substrate table WT are keptessentially stationary, while an entire pattern imparted to theprojection beam is projected onto a target portion C at once (i.e. asingle static exposure). The substrate table WT is then shifted in the Xand/or Y direction so that a different target portion C can be exposed.In step mode, the maximum size of the exposure field limits the size ofthe target portion C imaged in a single static exposure.

In a scan mode, the mask table MT and the substrate table WT are scannedsynchronously while a pattern imparted to the projection beam isprojected onto a target portion C (i.e. a single dynamic exposure). Thevelocity and direction of the substrate table WT relative to the masktable MT is determined by the (de)magnification and image reversalcharacteristics of the projection system PL. In scan mode, the maximumsize of the exposure field limits the width (in the non-scanningdirection) of the target portion in a single dynamic exposure, whereasthe length of the scanning motion determines the height (in the scanningdirection) of the target portion.

In another mode, the mask table MT is kept essentially stationaryholding a programmable patterning device, and the substrate table WT ismoved or scanned while a pattern imparted to the beam is projected ontoa target portion C. In this mode, generally a pulsed radiation source isemployed and the programmable patterning device is updated as requiredafter each movement of the substrate table WT or in between successiveradiation pulses during a scan. This mode of operation can be readilyapplied to maskless lithography that utilizes programmable patterningdevices, such as a programmable mirror array of a type as referred toabove. Combinations and/or variations on these various modes describedor entirely different modes of use may also be employed.

FIG. 2 shows a more detailed view of the source SO, illuminator IL andprojection system PL of FIG. 1. FIG. 3 shows an EUV radiationsystem/unit for use in the lithographic apparatus of FIG. 2. This hastwo plasma sources and a mirror for imaging the sources onto each other.More specifically, FIG. 5 shows two opposing plasma radiation sources 10and 12 that are optically open to each other, so that radiation can beimaged from one source onto the other. In particular, light can beimaged onto the first source 10 from the second source 12. Each source10,12 shown employs a gas or vapor, for example Xe gas or Li vapor, toform a very hot discharge plasma, so as to emit radiation in the EUVrange of the electromagnetic radiation spectrum. The sources 10, 12 mayeach include a pair of ionising electrodes (not shown) for causing apartially ionised plasma to collapse onto the optical axis O betweenthem. Alternatively, the plasma sources may be produced using a laser.In either case, partial pressures of 0.1 mbar of Xe, Li vapor, or anyother suitable gas or vapor may be required for efficient generation ofthe radiation. Techniques for producing radiation using a plasma areknown and will not be described in detail.

Between the plasma sources 10 and 12 is a single ellipsoidal mirror 14that is rotationally symmetric about the optical axis O and opticallyopen at both ends. The first source 10 is positioned so that a plasma isformed substantially at a first focal point of the mirror 14 and thesecond source 12 is positioned so that a plasma is formed substantiallyat the second focal point of the mirror 14. In this way, light generatedby the second source 12 can be focused towards and so imaged onto ormultiplexed with light generated by the first source 10. In order toprevent debris from the sources 10, 12 damaging or collecting on themirror 14, a debris suppression system (not shown), such as a foil trap,can be used in the near vicinity of each source 10,12 and between thatsources 10,12 and the mirror 14.

In use of the arrangement of FIG. 3, both sources 10, 12 can be fired atthe same time or the pulses can be interlaced. By interlacing, it ismeant that the sources 10, 12 are alternately fired, for example thesecond source 12 could be fired to produce a pulse of radiation and thenthe first source 10 could be fired. An advantage of interlacing thefiring sequences is that debris suppression systems are not neededaround the first source 10 when only the second source 12 is firing.Furthermore, any self-absorption of radiation produced by the secondsource 12 in the plasma of the first source 10 can be avoided, becausebetween pulses, the first source 10 is effectively transparent. Ineither case, by multiplexing the outputs of the two sources 10 and 12,an enhanced radiation output having an increased average power isprovided.

It should be noted that while FIG. 3 shows only two plasma sources 10and 12, more such sources could be used, each coupled to the othersusing the mirror arrangement of FIG. 3. Also, while a single ellipsoidalmirror 14 is used in the example of FIG. 3, it will be appreciated thatother suitable mirror arrangements could be used, such as a combinationof two parabloidal mirrors 16, 18, with a first source 20 beingpositioned to create radiation substantially at the focus of the firstparabloidal mirror 16, and a second source 21 being positioned so as tocreate radiation substantially at the focus of the second parabloidalmirror 18, as shown in FIG. 4.

In the arrangement of FIG. 4, the parabloidal mirrors 16 and 18 faceeach other and are rotationally symmetric about the axis O. To allowradiation to be transmitted in the illuminating direction, the end ofthe second parabloidal mirror 18 is optically open at the wavelength ofoperation. Because the first source 20 radiates at the focal point ofthe first parabloidal mirror 16, this means that radiation transmittedtowards the second source 21 is collimated, which in turn means thatwhen it hits the second mirror 18 it is focused towards the focal pointof that second mirror 18. In this way, radiation from the first source20 adds to the radiation from the second source 21 to provide anincreased power of the radiation. As will be appreciated, and as before,the first and second sources 20, 21 may be fired at the same time oralternatively their outputs could be interlaced.

Other options for the mirrors for imaging one radiation source ontoanother in accordance with the invention include a grazing incidencemirror, such as a Wolter-type mirror; a whispering gallery mirror, and aspherical mirror. These are well known and will not be described indetail.

FIG. 5 shows another arrangement for enhancing the output of a plasmaradiation source and providing an increased power beam. The arrangementis configured to image radiation from a single source 22 onto itself. Inthe arrangement, only a single electrical discharge plasma source 22 isprovided for producing illuminating radiation in an illuminatingdirection along the optical axis O. The source is open at its rear orback side. Behind the source 22 is a spherical mirror 24 for imaging thesource 22 onto itself. The mirror 24 is positioned so that the plasma isformed substantially at its focal point. Optionally, a debrissuppression system (not shown), such as a foil trap, is provided betweenthe source 22 and the mirror 24.

In use, when a plasma is generated by the source 22 of FIG. 5, light isradiated in all directions, some forwards in the illuminating directionand some rearwards. Because the source 22 is open at its rear end,radiation that is emitted rearwardly is directed towards the mirror 24,where it is reflected back to the focal point and from there forwardlyto add to the light being transmitted in the illuminating direction. Inthis way, the EUV optical output is significantly enhanced.

It will be appreciated that departures from the above-describedembodiments may still fall within the scope of the invention. Forexample, while the arrangements of FIGS. 3 and 5 are shown separately,it will be appreciated that these could be combined, as shown in FIG. 6.Also as an alternative to the simple mirror arrangement of FIG. 3, andto provide a good image of the second source 12 at the first source 10,the double ellipsoidal arrangement of WO 01/99143 could be used.Additionally, although the radiation sources of FIGS. 3-6 are describedwith reference to the lithographic apparatus of FIGS. 1 and 2, it willbe appreciated that these could be formed in separate, distinct unitsfor fitting to such an apparatus. Furthermore, although specificreference is made in this text to the use of lithographic apparatus inthe manufacture of ICs, it should be understood that the lithographicapparatus may have other applications, such as the manufacture ofintegrated optical systems, guidance and detection patterns for magneticdomain memories, liquid crystal displays (LCDs), thin film magneticheads, etc. It should be appreciated that in the context of suchalternative applications, any use of the terms “wafer” herein may beconsidered synonymous with the more general term “substrate”. Inaddition, the substrate referred to herein may be processed before orafter exposure, in for example, a track (a tool that typically applies alayer of resist to a substrate and develops the exposed resist) or ametrology or inspection tool. Furthermore, the substrate may beprocessed more than once, for example, in order to create a multi-layerIC, so that the term substrate used herein may also refer to a substratethat already contains multiple processed layers. Also, while thelithographic apparatus described includes a reflective reticle and aprojection system including reflective elements, a transmissive reticleand/or elements in the projection system may also be used. Furthermore,the apparatus has been described for use with EUV radiation but it willbe appreciated that radiation of other wavelengths may also be used.

While specific embodiments of the invention have been described above,it will be appreciated that the invention may be practiced otherwisethan as described. The description is not intended to limit theinvention.

1. A lithographic projection apparatus, comprising: an illuminationsystem configured to provide a beam of radiation; a support configuredto support a patterning device, the patterning device configured toimpart the beam with a pattern in its cross section; a substrate tableconfigured to hold a substrate, and a projection system configured toproject the patterned radiation onto a target portion of the substrate,wherein the illumination system includes a radiation source and at leastone mirror configured to increase the radiation power of the beam.
 2. Alithographic projection apparatus according to claim 1, wherein the atleast one mirror is provided behind the radiation source so thatradiation radiated rearwardly of the source is reflected forwardly toadd to the beam.
 3. A lithographic projection apparatus according toclaim 2, wherein the at least one mirror is a spherical or an asphericalmirror, and the radiation source is substantially at a focal point ofthe at least one mirror.
 4. A lithographic projection apparatusaccording to claim 1, further comprising a second radiation source,wherein the at least one mirror is configured to image the secondradiation source onto the radiation source.
 5. A lithographic projectionapparatus according to claim 4, wherein the at least one mirror isprovided between the radiation source and the second radiation source.6. A lithographic projection apparatus according to claim 5, wherein theat least one mirror comprises a single rotationally symmetric andoptically open-ended ellipsoidal mirror.
 7. A lithographic projectionapparatus according to claim 4, wherein the at least one mirrorcomprises two mirrors that are ellipsoidal and symmetrically arrangedalong the optical axis such that the second focal point of the firstellipsoidal mirror coincides with the first focal point of the secondellipsoidal mirror and the radiation source and the second radiationsource are arranged at first and second focal points, respectively, ofthe two ellipsoidal mirrors.
 8. A lithographic projection apparatusaccording to claim 4, wherein the at least one mirror comprises one of anested grazing incidence mirror, a Wolter-type mirror, a whisperinggallery mirror, and a spherical mirror.
 9. A lithographic projectionapparatus according to claim 4, wherein the at least one mirrorcomprises a pair of rotationally symmetric parabloidal mirrors that faceeach other, one of the radiation sources is positioned at a focal pointof a first one of the mirrors, the other one of the radiation sources ispositioned at a focal point of the other one of the mirrors, and one ofthe mirrors is optically open ended so that radiation can be transmittedthrough the open end in an illuminating direction.
 10. A lithographicprojection apparatus according to claim 1, wherein the radiation sourceis a plasma radiation source.
 11. A lithographic projection apparatusaccording to claim 10, wherein the plasma radiation source is anelectrical discharge source or a laser produced plasma source.
 12. Alithographic projection apparatus according to claim 1, wherein theradiation source is operable to emit radiation in the EUV range.
 13. Alithographic projection apparatus according to claim 1, furthercomprising a debris suppression system provided between the radiationsource and the at least one mirror.
 14. A device manufacturing method,comprising: providing a substrate; providing a beam of radiation usingan illumination system that includes a radiation source; patterning thebeam of radiation; projecting the patterned beam of radiation onto atarget portion of the substrate, and increasing a radiation power outputof the beam of radiation using at least one mirror in the illuminationsystem.
 15. A device manufacturing method according to claim 14, furthercomprising positioning the at least one mirror behind the radiationsource so that radiation radiated rearwardly of the source is reflectedforwardly to add to the beam radiation.
 16. A device manufacturingmethod according to claim 15, wherein the at least one mirror is aspherical or an aspherical mirror, and the source is substantially atthe focal point of the mirror.
 17. A device manufacturing methodaccording to claim 14, further comprising: providing a second radiationsource; and imaging the second radiation source onto the first radiationsource using the at least one mirror.
 18. A device manufacturing methodaccording to claim 17, wherein the at least one mirror comprises asingle rotationally symmetric ellipsopidal mirror.
 19. A devicemanufacturing method according to claim 17, wherein the at least onemirror comprises two mirrors provided between the sources for imagingthe second radiation source onto the first radiation source.
 20. Adevice manufacturing method according to claim 19, wherein the twomirrors are ellipsoidal, and the method further comprises: symmetricallyarranging the two mirrors along the optical axis such that the secondfocal point of the first ellipsoidal mirror coincides with the firstfocal point of the second ellipsoidal mirror; and providing theradiation source and the second radiation sources at the first andsecond focal points, respectively, of the two ellipsoidal mirrors.
 21. Adevice manufacturing method according to claim 17, wherein the at leastone mirror comprises a pair of rotationally symmetric parabloidalmirrors that face each other, one of the radiation sources is positionedat a focal point of a first one of the mirrors, the other one of theradiation sources is positioned at a focal point of the other one of themirrors, and one of the mirrors is optically open ended so thatradiation can be transmitted through the open end in an illuminatingdirection.
 22. A device manufacturing method according to claim 17,wherein the at least one mirror comprises one of a nested grazingincidence mirror, a Wolter-type mirror, a whispering gallery mirror, anda spherical mirror.
 23. A device manufacturing method according to claim14, wherein the radiation source is a plasma radiation source.
 24. Adevice manufacturing method according to claim 14, wherein the radiationsource is an electrical discharge plasma source or a laser producedplasma source.
 25. A device manufacturing method according to claim 14,wherein the radiation source is operable to emit radiation in the EUVrange.
 26. A device manufacturing method according to claim 17, furthercomprising causing the radiation sources to alternately emit radiation.27. A radiation system for providing a beam of radiation for alithographic projection apparatus, the system comprising: a radiationsource; and at least one mirror configured to increase the radiationpower of the beam.
 28. A radiation system according to claim 27, whereinthe at least one mirror is provided behind the radiation source so thatradiation radiated rearwardly of the source is reflected forwardly toadd to the beam.
 29. A radiation system according to claim 28, whereinthe at least one mirror is a spherical or an aspherical mirror, and thesource is substantially at the focal point of the mirror.
 30. Aradiation system according to claim 27, further comprising: a secondradiation source, wherein the at least one mirror is configured to imagethe second radiation source onto the radiation source.
 31. A radiationsystem according to claim 30, wherein the at least one mirror isprovided between the radiation sources to image the second radiationsource onto the radiation source.
 32. A radiation system according toclaim 30, wherein the at least one mirror comprises a singlerotationally symmetric ellipsoidal mirror.
 33. A radiation systemaccording to claim 30, wherein the at least one mirror comprises twomirrors, the two mirrors being ellipsoidal and symmetrically arrangedalong the optical axis such that the second focal point of the firstellipsoidal mirror coincides with the first focal point of the secondellipsoidal mirror, and the radiation source and the second radiationsource are arranged at first and second focal points, respectively, ofthe two ellipsoidal mirrors.
 34. A radiation system according to claim30, wherein the at least one mirror comprises a pair of rotationallysymmetric parabloidal mirrors that face each other, one of the radiationsources is positioned at the focal point of a first one of the mirrors,the other one of the radiation sources is positioned at the focal pointof the other one of the mirrors, and one of the mirrors is opticallyopen ended so that radiation can be transmitted through the open end inan illuminating direction.
 35. A radiation system according to claim 30,wherein the at least one mirror comprise one of a nested grazingincidence mirror, a Wolter-type mirror, a whispering gallery mirror, anda spherical mirror.
 36. A radiation system according to claim 27,wherein the radiation source is a plasma radiation source.
 37. Aradiation system according to claim 36, wherein the plasma radiationsource is an electrical discharge source or a laser produced plasmasource.
 38. A radiation system according to claim 27, wherein theradiation source is operable to emit radiation in the EUV range.
 39. Adevice manufactured according to the method of claim 14.